Re: [PATCH v2 4/4] arm64/syscalls: Specific usage of verify_pre_usermode_state

On Thu, Mar 9, 2017 at 4:23 AM, Mark Rutland [off-list ref] wrote:

We generally stick to lower case for the arm64 assembly macros. If we
need this, we should stick to the existing convention.

quoted

+/* Similar to set_fs(USER_DS) in verify_pre_usermode_state without a warning. */
+.macro VERIFY_PRE_USERMODE_STATE
+     mov     x1, #TASK_SIZE_64
+     str     x1, [tsk, #TSK_TI_ADDR_LIMIT]
+.endm

We need arm64's set_fs() to configure UAO, too, so this is much weaker
than set_fs(), and will leave __{get,put}_user and
__copy_{to,from}_user() able to access kernel memory.

We don't currently have an asm helper to clear UAO, and unconditionally
poking that on exception return is liable to be somewhat expensive.

Also, given we're only trying to catch this in syscalls, I'm afraid I
don't see what we gain by doing this in the entry assembly.

I optimized all architectures from the arm (32-bit) discussion. I will
come back to a simple bl to the verify function. Thanks!
-- 
Thomas